Universal ATUs for HF Ranges
A Universal ATU
Radio amateurs are always very interested in devices such as antennas and ATUs that are simple and universal in usage that are capable of working on all amateur HF bands. This type of antenna and equipment has been one of choice by the military for many years. In the next section we will examine the workings of such universal ATUs that were developed for and used by the military. An almost identical schematic was used in a broadband ATU found in some old Soviet Union information. It was an automatic ATU that could handle medium power up to 5kW in both civilian and military transmitters. Now similar matching devices are used in transmitters and can match automatically any antenna to the transmitter. Other similar types of ATU have been discussed in various amateur radio publications many times in the past. Let us now analyze one of these simple and effective amateur versions of the type shown in Fig. 1 and is one I have tested personally and found to be very effective.
Fig. 1 Universal ATU

This ATU is capable of matching the transmitter final amplifier stage of 50-70 ohms output to an antenna load of 20 to 300 ohms in the frequency range of 1.8-32 MHz with an SWR of no higher than 2:1.
How it works:
7 to 32 MHz:
For operation in the range of 7 to 30-MHz range, the circuit consisting of the lower section “A” of coil L1 and variable capacitor C2.3 is tuned to resonance and the middle section “B” serves as a coupling coil for section “A”. C1 capacitor also provides the fine matching to the final amplifier stage. In the higher frequencies the upper part of the coil L1 acts as an RF choke and does not interfere with part “A” and “B” sections of the coil.
For both ranges, the coupling coil L2 is a compromise for coupling to the antenna. By adjusting C1 and C2, satisfactory matching to the antenna can be achieved in both frequency ranges. This ATU does not cover the proposed new amateur range at 5 MHz. To use the ATU in the new amateur range, a tap at 3.5 turns in section “C” of coil L1 needs to be added as shown in Figure 2.
Fig. 2 Universal ATU for all amateur HF ranges

The effectiveness of this ATU
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut elit tellus, luctus nec ullamcorper mattis, pulvinar dapibus leo.
1. Section “A”: 7 turns covering 40 mm long section of the coil form
2. Section “B”: 3 turns covering 15 mm long section of the coil form
3. Section “C”: 7 turns covering 30 mm long section of the coil form
The coil form must be of a high quality material, either RF glass or RF ceramic. The wire used for L1 should be no less than 1-mm diameter (#18 AWG). The winding L2 is four turns and it is wound in the middle part of L1 and covers 40 mm. L2 can be wound using either stranded wire or solid wire with a diameter of 1.5-2 mm (#17-#12 AWG) for either type of wire used for this coil.
Fig. 3 Design of L1 and L2

My concept was to design and build an ATU that could match any load with any reactive component, particularly if the reactive component was close to or equivalent to the value of the active component.
To eliminate this problem, C3 was added to this ATU. The addition of C3 allows better adjustment of the antenna coupling.
The diagram of this improved ATU is shown in Fig. 4.
For the entire frequency range from 1.8 to 32 MHz, the addition of C3 allows the SWR to be no worse than 1.5:1 using either 50 or 75 ohms impedance coaxial cable.
This is with any resistive load of 20-300 ohms. SWR readings of 1.2:1 in the HF range from 1.8-7 MHz can be achieved.
Fig. 4 Improved Universal ATU

Further improvement of the ATU in the ranges from 18-32 MHz is required where the gain of an amplifier decreases because of increase in frequency. L3 is an additional coupling coil that is added and only works in these ranges. The diagram for this ATU is shown in Fig. 5. The coil is made of 3 turns of flexible wire with a diameter of 1.5 mm (#17 AWG). It is wound above coil L1 as shown in Figure. 6. This coil increases the matching range of the ATU considerably in the frequency range of 7-32 MHz. In the range of 18-32 MHz, the coupling coil L3 will allow tuning of SWR down to no worse than 1.2:1 with coaxial cable with a 50-ohm characteristic impedance
Fig. 6 Placement of L3

The ATU shown in Figure 5 is a true universal ATU. It has only one switch, S1 that can be a simple toggle switch and three capacitors. It is a simple but effective tuner.
These three tuning capacitors are used to tune up the ATU for minimum SWR from the antenna. To tune the ATU, a SWR meter is used between the antenna and transmitter (Figure 7a).
Another way to check the tuning is to use an RF voltmeter on the ATU output terminals as shown in Figure 7b. When tuning in the range 3.5-7.5 MHz, it is possible to find two spots where the ATU will show low SWR. This is due to a resonance either in coil L1 or only in part “A” of the drawing of that coil. In this case it is necessary to tune for maximum RF voltage on the antenna as in both instances SWR will be low.
Fig. 7 Adjusting the universal ATU


Since all ATU capacitors are working in the resonance mode they need a large clearance between the plates because of the high voltage that is present when operating. If the spacing is too close, arcing is likely to occur. Spacing should be no less than 0.5 mm for 50 watts input to the ATU. If they do break down on transmission, it may be necessary to avoid tuning the capacitors to total resonance. However, this will reduce the effectiveness of the ATU. It is also necessary to use high-quality variable capacitors with good contacts on the rotor. This is important to the usage of the ATU on the upper amateur HF ranges.
An air variable high-voltage capacitor, C2, may be used with the capacity range of 10 to 200 pF for the ATU. It allows the reduction of the dimensions when the ATU is constructed. In this case it becomes necessary to use a two-pole three-position switch instead of the S1 toggle switch. The schematic for this ATU is shown in Figure 8. In this ATU the C4 and C5 capacitors must be selected for operation in the ranges of 3.5-1.9 MHz.
Fig. 8 The universal ATU with 2-pole 3-position switch

The “C” part of L1 can have up to 15 turns for more effective ATU work on 1.9 and 3.5 MHz. For most effective ATU amateur radio results with the antennas, vary the tap from L1 to C1. This tap can be from 5 to 10 turns from ground (see Figure 9).
Fig. 9 Further universal ATU improvements

Any of these ATUs may be built using any of these schematics that are presented. The one selected will depend on the matching requirement for the range of operation chosen and the antenna used. The most universal ATU is shown in Figure 8. If one wishes to operate with a symmetrical antenna then it will be necessary to use two variable capacitors on the ATU output as is shown in Figure 10.
Fig. 10 Universal ATU with symmetrical output

Simple universal matching device
In the previous paragraph, we have examined a simple universal ATU for all amateur bands. Is it possible to improve this simple ATU? Of course!
The schematic of an improved simple ATU is shown in Figure 11. As can be seen from the diagram, this ATU is a conventional LC-tuned circuit. With switch S3 and capacitor C1, it can be tuned on frequency of any selected amateur band. Switch S1 allows adjustable coupling with the transmitter’s final stage. S2 provides the variable coupling to the antenna. By using S1, S2, S3, one may match just about any transmitter’s final amplifier to any antenna. The matching network works well in the HF range from 1.8 to 30 MHz with nearly all antennas. But, in the higher ranges above 14 MHz to 32 MHz, the efficiency is lower.
Fig. 11 Simple matching device

For the coil form for L1, I used a glass mustard jar with an OD of 55 mm and a metal screw-on lid. L1 consists of 30 turns of copper wire with a diameter of 1.5 mm, (#14AWG). The coil was wound over a 50-mm length on the jar. The first and last turns were held in place by using automobile epoxy. The first, second, and third taps were placed at 2, 3 and 4 turns. Then the next turns 3, 4, and 5 were placed at 6, 8 and 10 turns. Taps 6 and 7 are at 13 and 15 turns. Then the turns are uniform for four turns. Then, tap 8 is at 19 turns, tap 9 at 23 turns, tap 10 is at turn 27 and tap 11 is at 30 turns. C1 capacitor is a twin air variable capacitor from an old tube broadcast receiver. These capacitors can be used at powers of 100 watts in this ATU. The ATU also improves reception considerably.
To tune the ATU, an LED D1 is used. The ATU is tuned for maximum brilliance from the LED. The LED is located in the winding of the current transformer T1. For extremely fine-tuning, it one may use an RF meter such as described in reference [2] below. The current transformer is wound on a ceramic ring with the OD of 22 mm. I used the ceramic ring from an old tube socket. All the leads and center metal clamp is removed. Another type of plastic ring may be used, but it must be no larger than a 15- to 25-mm OD. The thickness may be from 5-15 mm. The first winding of the transformer is 3 turns and the second has 30 turns.
The first winding is wound with L1 coil’s wire that has a wire diameter of 1.5 mm (#14AWG).
The second winding is wound with wire of 0.3 mm (#27-#25 AWG). The current transformer and construction of the ATU is shown in Figure 12a. The front view is shown in Figure 12b.
Fig. 12 An improved simple ATU design


The coil wound on the mustard jar L1 is fastened to the case of the ATU using the metal cap as a base mount. From L1 the wires go to the switches. Each wire goes to the appropriate terminal of the switch and is soldered to it. The capacitor C1 is behind the switch and its calibration is located on the front panel.
Because the capacitor C1 has a short adjusting lead, it was lengthened with help of an old ballpoint pen (see Figure 13).
Fig. 13 Capacitor C1 with lead adjustment

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut elit tellus, luctus nec ullamcorper mattis, pulvinar dapibus leo.
References
[1] Peter Linsley (G3PDL) : HF SINGLE COIL “Z-MATCH” ATU // SPRAT p.76. AUTUMN 1993
[2] Igor Grigorov: RF-ammeter //www.antennex.com. Archive V.
Originally posted on the AntennaX Online Magazine by Igor Grigorov, RK3ZK
Last Updated : 4th January 2025